Precipitation pattern of hydrides in Zr-2.5Nb pressure tubes

The objective of this study is to investigate the precipitation pattern of hydrides in Zr-2.5Nb pressure tubes. Hydrogen of 60 or 450 ppm H was charged electrolytically to a Zr-2.5Nb pressure tube made by extrusion at 780 .deg. C, cold drawing of ∼ 25% and stress relief at 400 .deg. C for 24 h. The hydrides precipitated on the transverse normal section (TS) and longitudinal normal section (LS) planes of the Zr-2.5Nb tube were subjected to detailed examination macroscopically by an optical microscope and a scanning electron microscope and microscopically by a transmission electron microscope. The precipitation pattern of hydrides was different on each section plane; microscopically, the hydrides precipitated mostly in the longitudinal direction on the TS plane, but TEM examination of the hydrides revealed that these long hydrides consisted of arrays of small stacked hydrides growing only along the long axis of a α-Zr grain. On the LS plane of the Zr-2.5Nb tube, two kinds of precipitates were observed: one grew along the circumferential direction in the α-Zr grain and the other grew across several α-Zr grains in the radial direction. The residual stress distribution of the hydrided Zr-2.5Nb tube was determined by an X-ray analysis on the TS and LS planes. The precipitation pattern of hydrides was discussed in association with the distribution of residual stress on the section planes of the Zr-2.5Nb tube